It is generally thought that oxidative modification by hydrogen peroxide or lipid peroxide lead to the inactivation of various hemoproteins, including myoglobin, hemoglobin and cytochrome P-450. Recently, however, work reported by Dr. Y Osawa at the NIH showed that addition of 1-2.5 equivalents of hydrogen peroxide to myoglobin resulted in cross-linking of an intact heme moiety to the protein. This altered hemoprotein has been shown to exhibit an oxidase-like activity. The alterations of myoglobin to an enzyme that can form toxic oxygen metabolites may have pathological importance, especially in myocardial injury caused by ischemia and reperfusion. This prompted us to ask the question of whether chemically modified hemoglobins developed as blood substitutes undergo similar structural and/or enzymatic modifications which may explain, at least in part, some of the toxicities associated with the infusion of hemoglobin- based products such as vasospasm and reoxygenation injury. Results transpired so far from our recent collaborative work with Dr. Osawa indicate that treatment of hemoglobin with low levels of hydrogen peroxide produced soluble protein-bound heme products that are chromatographically similar to that of myoglobin. Moreover, hemoglobins cross-linked at alpha subunits showed a typical propensity to oxidative modification than other forms of hemoglobin modifications. The heme derived adducts from the reaction f hydrogen peroxide with hemoglobin showed little or no enzymatic activity in the NADPH-diaphorase methemoglobin reductase system. Work is under way to test for enzymatic activity in a variety of physiologically relevant reducing systems.